Aluminum alloy



Filed Oct. 27 19:9

De c. 31, 1940.

INVENTOF Philip T Y wm Siroup ORNEY Patented Dec. 31, 1940 2,226,594 ALUMINUM ALLOY Philip T. Stroup, New Kensington, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Application October 27, 1939, Serial No. 301,594

7 Claims.

This invention relates to aluminum base alloys, and it is particularly concerned with controlling the grain size in castings.

Among the factors which affect the properties and behavior of both wrought and cast aluminum base alloy articles, one of the most important is the grain size of the metal. The term grain size refers to the dimensions of the individual crystals which compose the metallic body. The

grain size is usually referred to as being fine,

medium, or coarse, and the shape of the grains is described as being equi-axed or elongated, depending upon the relative dimensions of the grain. Generally, a fine equi-axed grain size is considered to be most desirable in an alloy both from the standpoint of strength and hardness, as well as workability. Since some aluminum base alloys do not inherently exhibit a small grain size in the as-cast condition, and, furthermore, since thermal conditions during solidification of the molten metal exercise such agreat influence upon the size of grains, it is necessary to exercise some control of the alloy composition or freezing conditions in order to insure a uniform structure in the product. This need is most apparent in the case of ingots and other castings which have cross sections of considerable thickness, because the slow cooling tends to promote the development of large grains.

A satisfactory means for controlling the size of grains in aluminum base alloy castings should possess the following characteristics: (1) convenience in application; (2) unlformity in effect;

and (3) a minimum of undesired effect on other important properties.

it is the principal object of my invention to provide a simple means for producing small equiaxed grains in cast aluminum base alloys. Another object is to provide a means for effecting this control of grain size which has the abovementioned characteristics. These and other objects will become apparent from the following description of my invention.

I have discovered that the addition of small amounts of one or both of the elements, columbium and tantalum, to aluminum base alloys produces a small grain size in the as-cast product.

While the presence of either element alone in an alloy has a pronounced effect upon the grain size, I

an even greater effect is obtained it both elements are present. As far as I have observed, the addition of these elements to aluminum base alloys does not adversely affect other properties which are generally desired, such as hardness, strength, ductility, workability, and resistance to corrosion. I have also observed thatthe grain-refining effect obtained through the addition of these elements is substantially uniform throughout the entire article. This uniformity in effect is particularly advantageous in the casting of ingots or other articles of relatively large cross sectional dimension.

The benefit derived from adding columbium and/or tantalum to aluminum base alloys as mentioned hereinabove is particularly evident in the reduction of the grain size of theas-cast metal. However, the addition of these elements may'also have other beneficial effects both in the casting and in the wrought product made from the cast article. By emphasizing the effect upon the grain size of the cast alloys, I do not wish to minimize any advantages gained in other respects.

Only relatively small amounts of columbium and tantalum are required to produce a fine grain size in castings, from 0.01 to 0.1 per cent of either one generally being suiiicient for the purpose. In certain cases it may be necessary to employ even more, but in no event should the amount exceed 0.5 per cent, and preferably not over 0.4 per cent. Although either element is effective Whenused separately, I have found that an even more pronounced grain-refining effeotis obtained if both are simultaneously employed. In such a case the. total amount should not be less than about 0.02 per cent, nor should it'exceed about 0.5 per cent.

The elements columbium and tantalum, for the purposes of my invention, are regarded as being equivalent to each other, that is, one may be substituted for the other although not necessarily in the same proportions, and therefore they constitute. a group. In addition to having a similar grain refining effect on aluminum base alloys, these elements resemble each other in that both of them occur in the same subgroup of group V of the periodic table, both have body-centered space lattices, and both form the same type of alloy constitutional diagram with aluminum.

The aluminum base alloys which are particularly benefited by the addition of at least one of the elements of the columbium-tantalum group are those containing from 0.25 to 12 per cent copper, or 0.5 to 15 per cent magnesium, or 0.25 to 14 per cent silicon, or 0.5 to 20 per cent zinc, or 0.1' to 3 per cent manganese, or combinations of two or more of these elements. These alloys may also contain one or more of the following elements, 'often'referre'd to as hardeners, in the following percentages: 0.05 to 0.5 per cent chromium, 0.01 to 0.5 per cent titanium, 0.25 to 2.5 percent nickel, 0.01110 0.5 per cent boron, 0.002 to 2 per cent beryllium, 0.1 to 0.5 per cent molybdenum, and 0.1 to 0.5 per cent zirconium. The total amount of the latter'elements, however, should not exceed about 3 per cent. As exemplary of the variety of alloys whose grain size has been found to be reduced by the addition of columbium and/or tantalum, the following compositions are cited, wherein aluminum constitutes cent manganese; 2.5 per cent magnesium, 0.25

iii?

per cent chromium; 2 per cent M8281, 0.25 per cent chromium; 4 per cent copper; 5 per cent silicon; 5.25 per cent MgZnz; 1.25 per cent magnesium, 0.5 per cent zinc, 0.15 per cent copper; and 4.4 per cent copper, 0.65 per cent manganese, 1.5 per cent magnesium.

The effect of adding columbium or tantalum, or both elements, to a particular alloy is illustrated in the accompanying figures, where Fig. 1 is a photomicrograph of an as-cast a1 loy composed of 2.5 per cent magnesium, 0.25 per cent chromium, the balance commercially pure aluminum;

Fig. 2 is a photomicrograph oi the samealloy to which 0.03 per cent columbium had been added;

Fig. 3 is a photomicrograph of the same alloy to which 0.06 per cent tantalum had been added; and

Fig. 4 is a photomicrograph of the same alloy to which 0.02 per cent columbium and 0.07 per cent tantalum had been added.

The alloy employed for the test was one which is widely used in wrought form, and has a nominal composition of 2.5 per cent magnesium, 0.25 per cent chromium, and the balance aluminum containing a maximum of 0.3 per cent iron and silicon as impurities. A quantity ofithe alloy was first melted and a specimen poured at a temperature of 1350 F. into a cold, thin-walled iron mold having the the shape of a frustum of an inverted cone with a diameter of about three inches at the base of the cone. About five minutes was required for the metal to completely solidify, which tended to promote the formation of large grains. The remaining melt was divided into three portion-s, 0.03 per cent columbium tie-- ing added to one, 0.06 per cent tantalum being added to the second, and ii.02 per cent columbium and 0.07 per cent tantalum being doc! to the third. Specimens were cast at a temperature of 1350 F. in the same iron mold as the alloy with out the columbium or tantalum additions, the mold in each case being at room temperature, or "cold," when the metal was poured into it. The specimens were sectioned in a vertical plane, poi= ished, and etched in an aqueous solution of nitric and hydrochloric acids. A representative section of each specimen was then photographed at a magnification of three diameters.

In Fig. l the large grains of the untreated alloy may be plainly seen. Grains of this size are regarded as being too coarse for a satisfactory casting as well as promoting cracking and checking in a body that is to he subsequently worked. The criss-cross markings on some of'the grains illustrate a common solidification phenomenon known as dendritic formation. The grain-refining eiiect, of adding columbium to the alloy is seen in Fig. 2. In comparison to Fig. l, the

grains are very small and soul-axed. In Fig. 3, the grain size of the alloy to which tantalum had been added may be seen. Since 0.06 per cent tantalum was employed, as compared to 0.03 per cent columbium in the preceding example, it is not surprising that the grain size should be smaller than in Fig. 2. The very marked effect of both columbium and tantalum on the grain size is shown in Fig. 4. The grain size is so small as to be scarcely distinguishable at a magnification of three diameters, which is the same magnification that was used in the other photomicrographs.

The tantalum and columbium may be added to molten aluminum base alloys in any convenient manner. I have found that the ferro-alloys of these two elements provide a satisfactory source. The ferro-alloy is preferably diluted with aluminum at a high temperature, and this diluted alloy containing, for example, 2 to 5 per cent of columbium or tantalum is used for making additions. This diluted alloy may ,be referred to as a hardening or rich alloy. Generally speaking, since the amount of tantalum and columbium used is so small, the amount of iron which is also introduced along with these elements from the ferro-alloys is likewise small and has no significant efi'ect in the case of most alloys. Another advantage obtained through using the ferro-alloys as a source of columbium and tantalum is that both of these elements will usually be present and therefore tend to produce an even finer structure than if only one is employed.

In referring to aluminum base alloys herein, I

mean those which contain at least 50 per cent aluminum. The term aluminum as herein employed refers to the metal as commerecially pro- 4 duced which contains impurities.

I claim:

l. A cast aluminum base alloy containing from 0.5 to 20 per cent zinc and 0.01 to 0.5 per cent of at least one of the metals of the group composed of tantalum and columbium, and characterized by a fine grain size as compared to the same alloy devoid of tantalum and columbium.

2. A cast aluminum base alloy containing from 0.5 to 15 per cent magnesium, 0.5 to 20 per cent Zinc, and 0.01 to 0.5 per cent of at least one of the metals of the group composed of tantalum and columbium, and characterized by a fine grain size as compared to the same alloy devoid of tantalum and columbium.

3. A cast aluminum base alloy containing from 0.5 to 20 per cent zinc, 0.25 to 14 per cent silicon and from 0.01 to 0.5 per cent of at least one of the metals of the group composed of columbium and tantalum, and characterized by a fine grain size as compared to the same alloy devoid of tantalum and columbium.

i. A cast aluminum case alloy containing from 0.5 to 20 per cent zinc, 0.1 to 3.0 per cent manganese and from 0.01 to 0.5 per cent of at least one of the metals of the group composed of columbium and tantalum, and characterized by a fine grain size as compared to the same alloy devoid of tantalum and columbium.

5. A cast aluminum base alloy containing from 0.5 to 20 per cent zinc, 0.5 to 15 per cent magnesium, and at least 0.01 per cent of each of the metals columbium and tantalum, the total amount of said two metals not exceeding 0.5 per 'cent, the balance of the alloy being aluminum.

columbium and tantalum, the total amount of said two metals not exceeding 0.5 per cent, the balance of the alloy being aluminum.

PHILIP T. STROUP. 

